1. Field of the Invention
The present invention is related to magnetofluidic acceleration sensors.
2. Background Art
Magnetofluidic accelerometers are generally known and described in, e.g., U.S. patent application Ser. No. 10/836,624, filed May 3, 2004, U.S. patent application Ser. No. 10/836,186, filed May 3, 2004, U.S. patent application Ser. No. 10/422,170, filed May 21, 2003, U.S. patent application Ser. No. 10/209,197, filed Aug. 1, 2002 (now U.S. Pat. No. 6,731,268), U.S. patent application Ser. No. 09/511,831, filed Feb. 24, 2000 (now U.S. Pat. No. 6,466,200), and Russian patent application No. 99122838, filed Nov. 3, 1999 that utilize magnetofluidic principles and an inertial body suspended in a magnetic fluid, to measure acceleration. Such an accelerometer often includes a sensor casing (sensor housing, or “vessel”), which is filled with magnetic fluid. An inertial body (inertial object) is suspended in the magnetic fluid. The accelerometer usually includes a number of drive coils (power coils) generating a magnetic field in the magnetic fluid, and a number of measuring coils to detect changes in the magnetic field due to relative motion of the inertial body.
When the power coils are energized and generate a magnetic field, the magnetic fluid attempts to position itself as close to the power coils as possible. This, in effect, results in suspending the inertial body in the approximate geometric center of the housing. When a force is applied to the accelerometer (or to whatever device the accelerometer is mounted on), so as to cause angular or linear acceleration, the inertial body attempts to remain in place. The inertial body therefore “presses” against the magnetic fluid, disturbing it and changing the distribution of the magnetic fields inside the magnetic fluid. This change in the magnetic field distribution is sensed by the measuring coils, and is then converted electronically to values of linear and angular acceleration. Knowing linear and angular acceleration, it is then possible, through straightforward mathematical operations, to calculate linear and angular velocity, and, if necessary, linear and angular position. Phrased another way, the accelerometer provides information about six degrees of freedom—three linear degrees of freedom (x, y, z), and three angular (or rotational) degrees of freedom (angular acceleration ω′x, ω′y, ω′z about the axes x, y, z).
Generally, the precise characteristics of the acceleration sensor are highly dependent on the geometry of the housing, the inertial body, the arrangements of the magnets, the properties of the magnetic fluid, etc. For a designer, as wide a range as possible of various sensor parameters is desirable. Such parameters include, e.g., dynamic range, sensitivity, response time, physical dimensions, cost, drift, susceptibility to environmental factors, etc. One of the factors that effects the performance of the sensor is hydrodynamic resistance, which results from the inertial body trying to move against the magnetic fluid. Generally, the magnetic fluid is a relatively viscous fluid, and the larger the area of the inertial body in contact with the magnetic fluid, the greater the hydrodynamic resistance. Higher hydrodynamic resistance therefore leads to a lower frequency response.
Accordingly, there is a need in the art for a way to reduce hydrodynamic resistance in a magnetofluidic accelerometer.